As a highly efficient energy resource, petroleum plays an irreplaceable role in the national economy. As the national economy in China grows rapidly, the demand for petroleum is increasing continuously. In year 2015, the domestic yield of crude oil was 213 million tons, and the quantity of imported crude oil was 334 million tons, and the foreign-trade dependency was as high as 62%. Hence, efficient exploitation of petroleum is a major demand and important safeguard for national energy security.
As oil and gas exploration and development is deepened, the percentage of exploration and development of low-permeability oil and gas resources has become higher and higher. The average exploration and development percentage of low-permeability oil and gas resources has accounted for about 67% of newly increased proven reserves during the 12th 5-year plan; especially in recent years, new proven ultra-low permeability oil reservoirs were especially abundant. In China, ultra-low permeability oil and gas resources have been regarded as a major domain of national oil and gas development.
The oil recovery of ultra-low permeability oil reservoirs recovered by means of natural energy is usually lower than 10%. To improve the oil recovery of those oil reservoirs, the oil reservoir energy has to be supplemented by water or gas injection. However, in a water injection process, a hydrated film may be formed easily, and the clay minerals in the formation swell and the pores tend to be closed when the clay minerals encounter water, resulting in rapidly increased injection pressure and severely decreased injection volume or even failure of injection; consequently, the recovery efficiency of the matrix is very low. Compared with water injection, injected gas is easier to permeate into micro-nano pores, and can effectively displace the crude oil in the matrix; therefore, the oil displacement efficiency can be remarkably improved. Especially, carbon dioxide is usually more dissoluble in water than hydrocarbon gasses, and has higher dissolvability in crude oil than in water. Carbon dioxide dissolved in a water solution can transfer to dissolve in crude oil, and has advantages including high mobility, viscosity reduction, volumetric expansion, interfacial tension reduction, plugging removal by acidization, light hydrocarbon extraction, and easy phase-mixing features, etc. Therefore, carbon dioxide flooding has received extensive concern as an effective method for enhancing oil recovery for low-permeability oil reservoirs in China and foreign countries, and has achieved favorable effects in applications in oil fields. However, oil reservoirs with ultra-low permeability are usually accompanied with natural and/or artificial fractures. As a result, after carbon dioxide is injected into the formation, since the viscosity of carbon dioxide is much lower than that of water, gas channeling may occur easily along the fractures; therefore, the swept volume of carbon dioxide and the oil displacement efficiency are severely decreased, so that the field experiment result is severely degraded. In recent years, domestic and foreign researchers have put forward 5 types of mobility control systems that can be used in the carbon dioxide flooding process and associated methods:
Water Alternating Gas (WAG)
The Water Alternating Gas (WAG) method is the most widely applied and most successful carbon dioxide mobility control method. WAG is to inject water slugs and gas slugs alternatively to the oil reservoir, wherein, water firstly enters into the fractures and form a shield; since a phase interface exists between water and gas, the water saturation is increased, while the gas saturation is decreased, and thereby the relative permeability of carbon dioxide is decreased, the gas transfers into the matrix, the gas-to-oil mobility ratio is improved, the swept volume of gas is expanded, and the purpose of improving oil recovery is attained. However, for oil reservoirs with ultra-low permeability, the follow-up injection is very difficult, and the introduced water will hamper carbon dioxide from mixing with hydrocarbon compounds.
Foam
A foamed system has a “selective plugging” characteristic. The foams can effectively decrease the relative permeability of gas in the porous media, and thereby can effectively control gas mobility in the carbon dioxide injection process; in addition, foam flooding applied in oil fields is relatively successful. The foam flooding effect generated by injecting a surfactant and a gas simultaneously is more advantageous than that generated by injecting the surfactant and the gas alternatively; in addition, by comparing the effect of foam flooding in improvement of vertical sweep efficiency with the effect of polymer flooding, it is concluded that the effect of foam flooding for high-permeability zones is superior to that of polymer flooding. To improve stability of foams, polymer enhanced foam, gelled foam, and three-phase foam are developed. However, foaming agents employed by conventional foam flooding are usually water-dissoluble surfactants. When such a surfactant is used for ultra-low permeability oil reservoirs, the solution of water-dissoluble foaming agent is difficult to inject, and consequently it is difficult to use the foam fluid to control carbon dioxide mobility.
Polymer Viscosifiers
Polymers can be directly dissolved as chemical additives in supercritical carbon dioxide to attain an effect of increasing the viscosity of the supercritical carbon dioxide. Domestic and foreign researchers have studied the influences of supercritical carbon dioxide fluid admixed with polymer viscosifiers on mobility control and sweep efficiency through laboratory experiments and field experiments. Heller et al. has found that polymers can be much more dissolved as mobility control agents in supercritical carbon dioxide, and the structure of polymer, chemical properties of crystal, and molecular weight, etc. have significant influence on the solubility of polymers in carbon dioxide. At present, polymers that have been studied extensively include fluoropolymers and ordinary polymers. Wherein, ordinary polymers are hydrophobic and usually have a problem of poor solubility, which results in a poor viscosifying effect; fluoropolymers have much better dissolvability in supercritical carbon dioxide, and can attain a fairly good viscosifying effect; however, such polymers have their obvious shortcomings: firstly, the production cost is too high to mass production; secondly, the environmental hazard is severe, and adverse to environmental protection.
Gel
The channel plugging mechanism of gel is to utilize a gel solutions to form gel for plugging in the fractured matrix or channels. With reference to gel materials for conformance control, a gel system (e.g., sodium silicate gel) that matches the oil reservoir can be developed according to the actual oil reservoir conditions; specifically, water glass gel generated through a reaction between sodium silicate solution and carbon dioxide is utilized to inhibit carbon dioxide channeling and attain the purpose of achieving mobility control in carbon dioxide flooding. The viscosity of the system is low and equivalent to the viscosity of water; hence, the system has a characteristic of high plugging strength; however, a certain degree of contamination to the matrix of the ultra-low permeability oil reservoir will be resulted.
Precipitation
The basic plugging principle of the precipitation method is to control a salt solution (e.g., magnesium salt, calcium salt, or barium salt) hydrolyzed to an alkaline state or an organic amine (ethylene diamine) to react with injected carbon dioxide to generate carbonate precipitation, and thereby attain an plugging effect. The chemical precipitation method can effectively improve the mobility of carbon dioxide, and can improve the sweep efficiency by about 20-30%. However, it is necessary to note that the pH value of the solution in the actual oil reservoir or the reaction of NaOH with rocks in the oil reservoir will make it difficult to control the pH reasonably during the construction process. Ethylene diamine belongs to small molecular organic amine, and is inflammable, toxic, and harmful to health and environment in itself; in addition, if the oil saturation is high, the plugging strength of the generated precipitate will be decreased.
Though the above-mentioned methods have certain carbon dioxide mobility control capability, they have drawbacks such as unsatisfactory effect, complex operation, environmental pollution, and formation damage, etc.